1. Field of the Invention
The present invention generally relates to charged particle beam tools and, more particularly, to systems for locating an electron beam with high positional accuracy on a target in a shaped beam lithography tool.
2. Description of the Prior Art
Lithography is a technique utilized, for example, in the manufacture of semiconductor integrated circuits. While increases in performance, functionality and manufacturing economy have been derived from increased integration density, many additional techniques have been developed at much smaller sizes than can be resolved lithographically, a lithographic process is basic to determining the location and basic dimensions of both electrically active (e.g. transistors) and passive (e.g. conductors, storage capacitors) devices on a wafer which is later diced into individual chips. Therefore, increases in integration density have required that minimum lithographic feature size dimensions be reduced dramatically. At the present time, minimum feature size regimes are required which cannot be reliably produced with electromagnetic radiation (e.g. deep ultraviolet (DUV) light). Therefore, charged particle beams have been used for lithographic resist exposure when features below about 0.25 microns must be resolved.
Electron beam exposure tools have become the lithographic resist exposure tool of choice for such small minimum feature size regimes. However, increased integration density implies increased integrated circuit complexity and multiple exposures are generally required to develop the desired lithographic pattern for the chip. Such multiple exposures must be made with extreme positional accuracy and at high repetition rates to develop adequate throughput comparable to that of visible light or DUV resist exposure tools.
The most basic of electron beam lithography exposure tools is referred to as a probe-forming tool which is structurally similar to a cathode ray tube and capable of exposing only a single pixel at a time. Such a tool does not have an acceptable level of throughput for manufacturing applications with patterns that may include billions of pixels and is generally used only for extremely small production runs of application-specific integrated circuits (ASICs) of substantially unique design. To greatly increase throughput, electron-beam-projection lithography tools have been developed which allow exposures of sub-fields containing perhaps several million pixels by passing a relatively broad electron beam through a patterned mask so that the pattern of the mask is projected on the target. Between these extremes are shaped-beam-lithography tools which shape only the outer perimeter of a relatively broad beam to simultaneously expose several dozen to several hundred pixels at a time.
Shaped-beam-exposure tools may provide shaping of the beam in several ways. For example, a mask can be provided with numerous desired shapes and the mask moved to place an aperture of a desired shape in the beam path in a manner similar to an electron beam projection lithography tool. However, the settling time for mechanical movement of the mask reduces the maximum exposure rate that can be achieved and this limitation is very significant in view of the relatively small number of pixels which can be simultaneously exposed.
To overcome this limitation, shaped-beam-exposure tools have been developed which use two (or more) aligned, generally square, shaping apertures with a deflection arrangement there between. Thus a beam shaped by a first shaping aperture is caused to overlay (and be intercepted by) only a portion of the second shaping aperture in order to develop, for example, rectangular shapes of desired aspect ratios. Such a system, while avoiding mechanical movement of parts, presents problems of positioning since the shaped beam will be asymmetrically located with respect to the original beam and include stationary edges formed by the second shaping aperture and variable edges formed by the first aperture. Therefore, complementary deflection after shaping will result in the beam being off-axis, To avoid undue complication in achieving a desired positioning the beam at the target, a corner of the shaped beam formed by the second shaping aperture is used as a reference point.
In this regard, it should be understood that, in all types of electron-beam-exposure tools, while positional errors in beam location may be sensed and corrected by a feedback arrangement, beam positioning is xe2x80x9copen loopxe2x80x9d at the point that the actual exposure is made since the beam position cannot be sensed in a manner which is consistent with the resist exposure at the instant the exposure is made. Therefore, historically, accurate placement of the beam during exposure with a probe forming lithography tools has relied on the stability of the probe forming tool after it has undergone or performed some calibration or registration process. In the best of probe forming systems, the probe beam is caused to scan a sparse array of targets located on the substrate either initially and/or periodically between lithographic pattern exposures. The time between scanning of the targets thus xe2x80x9ccloses the loopxe2x80x9d in providing feedback to the system to correct the probe beam location although the operation of such an arrangement is not continuous and relies to some degree on system stability. That is, at the time of exposure, no additional, real-time measurement of beam location error or beam position correction can be performed.
For this purpose, use of a scintillating grid as a sparse target array has been suggested in xe2x80x9cExtending Spatial-Phase-Locked Electron-Beam Lithography to Two Dimensionsxe2x80x9d by Goodberlet et al., Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 7557-7559, which is hereby fully incorporated by reference. Light output from the scintillating grid is detected during beam scanning to determine points in time when the electron beam in incident thereon. Difference of color of light output and difference of spatial frequency of grid lines are suggested for distinguishing between coordinate directions. However, the techniques for obtaining beam position information described therein and in U.S. Pat. No. 5,136,169 by H. I. Smith rely on the narrowness of the probe beam in probe-forming electron-beam tools and thus are not applicable to charged particle beam tools having a broad beam as is required for acceptable production throughput.
Further, there is an additional class of lithographic exposures known as mask making (e.g. for making reticles for use in any optical, deep UV, extreme UV, electron-projection, ion-projection, and x-ray lithography tools), which does not allow for placement of registration targets on the workpiece or otherwise in the target plane. For this class of exposures, the pattern placement accuracy depends on a process known as emulation whereby the patterned substrate is characterized by an external metrology tool and the positional error information is fed back to the tool for subsequent exposures. This mode of operation is known as blind writing and the time between external measurement and subsequent corrected exposure may be measured in days. This mode of operation only succeeds when the tool possesses extreme stability.
It should be appreciated from the foregoing that the conventional practice of electron-beam exposures complicate or, in the case of mask making, prevent positional correction or compensation at the time of the exposure. Further, known feedback arrangements such as that disclosed in the above-incorporated article and U.S. Pat. No. 5,136,169 are limited to use with probe-forming tools that provide a single, small, round-exposure spot scanned in a rasterized fashion, and are not applicable to electron-beam tools which use a broader shaped or patterned beam to provide increased throughput. At the same time, high positional accuracy of electron beam location is often even more critical to properly stitch together sequentially exposed features than in a probe-forming-electron-beam tool. For example, in shaped beam or other electron-beam-projection tool exposures, positioning accuracy at the edge of the exposure should be held to a very small fraction of the minimum feature size to prevent, for example, excessive narrowing of a structure such as a conductor where adjacent exposures are joined.
It is therefore an object of the present invention to provide a spatial phase locking system for achieving accurate beam positioning which can provide continuous beam position correction and which is operable with charged-particle-beam tools which produce a broad beam, such as shaped beam and electron-beam-projection lithography tools.
It is another object of the invention to provide a reduction of the requirement of extreme stability or criticality of charged particle-beam-exposure tools, particularly of the shaped-beam and beam-projection type.
In order to accomplish these and other objects of the invention, a method of operating a charged particle beam tool is provided including steps of dithering a shadow pattern relative to and within a charged particle beam, and detecting incidence of said shadow pattern on a sparse array of targets.
In accordance with another aspect of the invention, a method of operating a charged particle beam lithography tool is provided including steps of causing a moving shadow pattern within a shaped or patterned charged particle beam, deflecting the shaped or patterned charged particle beam to a desired location on a target, and correcting deflection based on a time of incidence of the moving shadow pattern on fiducial marks on the target.
In accordance with a further aspect of the invention, a charged particle beam lithography tool including a source of a beam of charged particles, a grid for causing a shadow pattern within the charged particle beam, a deflector for dithering the shadow pattern, an aperture for shaping said charged particle beam, a deflector for deflecting said charged particle beam to a desired location on a target including fiducial marks, a detector for detecting when said dithered shadow pattern is incident on said fiducial marks, and an arrangement for generating a deflection correction in response to said means for detecting.